Hydraulic hardened foamed product and a method of producing the same

ABSTRACT

A method of making a composite product such as a building board, includes the steps of mixing together finely divided lignocellulosic fibres, a hydraulic binder, and water optionally containing a polyvinyl alcohol, to form a paste; introducing a foam generated from a polyvinyl alcohol into the paste and mixing to form a foamed product; forming the foamed product into a desired shape; and allowing the hydraulic binder to set to form the composite product.

This application is the national phase of international applicationPCT/GB98/03556 filed Nov. 27, 1998 which designated the U.S.

BACKGROUND OF THE INVENTION

This invention relates to a method of making a composite product from afinely divided lignoccllulosic material and a suitable hydraulic binder,and to the composite product so made.

Cement bound wood wool and cement bound particle board are well known.Cellular or aerated cents are equally well known, the cells being formedeither by foaming agents in water, the foam being added to a cementpaste, or alternatively the cells being formed in the cement by an agentevolving gas in an alkaline medium.

An example of a foamed building board is disclosed in South AfricanPatent No 92/6179 (corresponding to U.S. Pat. No. 5,395,571). Thispatent teaches a method of making a foamed building board or the likefrom the following components:

(a) a major amount of weight of an inorganic base material selected fromthe group consisting of a calcium sulphate hemihydrate, magnesiumoxychoride, magnesium oxysulphace and a hydraulic cement;

(b) a suitable amount by weight of the inorganic base material of athermosetting resin which is miscible, soluble or dispersable in water;

(c) a suitable amount of a catalyst for the thermosetting resin:

(d) water in an amount sufficient to rehydrate the inorganic basematerial with the water present in the other components;

(e) optionally a suitable amount of a plasticizer such as a melamineformaldehyde condensate;

(f) optionally a suitable amount of a polyvinyl alcohol;

(g) optionally a suitable amount of a retarder for the setting time ofthe inorganic base material;

(h) optionally a suitable amount of a fibrous reinforcing material; and

(i) a suitable amount by weight of a foam or a foaming agent;

which method comprises the steps of:

(1) mixing together components (a), (b), (c) and (d), and (e), (f) and(g) if present;

(2) adding component (h) if present into the mixture of step (1);

(3) adding component (i) into the mixture of step (2) with stirring togive a foamed product;

(4) forming the product of step (3) into a building board; and

(5) allowing rehydration of the inorganic base material to occur andcuring of the thermosetting resin to occur.

Cement bound lignocellulosic composites can suffer from the disadvantagethat the celluloses and the hemi celluloses in the lignocellulosicmaterial are attacked by the alkaline components of the cement, andsoluble sugars can retard Portland cement hydration. In addition, infoamed or cellular cement composites, cracking due to shrinkage andexcessive brittleness can occur. A further disadvantage of cellularcement bound lignocellulosic composites is the cost of the foamingagent. Further, the extended setting time of, for example, a Portlandcement can result in partial or total subsidence of the heavierfractions in the foamed mix resulting in variable cell size, or evenmajor voids. Difficulty in process parameter controls can result in aproduct of variable density, consistency and performance.

There is a need for a new composite product containing a lignocellulosicmaterial and a hydraulic binder.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof making a composite product including the steps of:

(a) mixing:

(i) a hydraulic binder,

(ii) finely divided lignocellulosic fibres in an amount of from 1% to30% inclusive by mass of the hydraulic binder; and

(iii) water optionally containing a polyvinyl alcohol, the water beingpresent in an amount sufficient to form a paste;

(b) introducing a foam generated from a polyvinyl alcohol into thepaste, in an amount of from 0.5% to 15% inclusive by mass of the pasteand mixing to form a foamed product;

(c) forming the foamed product into a desired shape; and

(d) allowing the hydraulic binder to set to form the composite product.

It is to be noted that the composite product is formed in the absence ofa thermosetting resin. In other words the composite product contains nothermosetting resin.

Lignocellulosic material refers to any plant material emanating from bephotosynthetic phenomenon. This includes paper, linen, cotton, hessian,and the like.

By finely divided lignocellulosic fibres, there is meant unifibres, i.esince fibres, or bundles of a small number of unifibres of thelignocellulosic material. In other words, the lignocellulosic materialis broken down into single or unifibres or bundles of a small number offibres, rather than being in chip or particle form. This is necessary asthe function of the finely divided lignocellulosic fibres is to act as atheological controller and foam stabiliser, and as a reinforcing fibrestrengthened by the polyvinyl alcohol.

The finely divided lignocellulosic fibres are preferably obtained frompaper mill sludge, paper waste or refined fibres such as are used inmedium density fibreboard manufacture.

The hydraulic binder may be selected from the group consisting of ahydraulic cement, such as a Portland cement e.g ordinary Portland cementor Rapid Hardening Portland cement, a calcium sulphoaluminate cement, ahigh alumina cement, a gypsum cement, cacium sulphate hemihydrate ineither the alpha or beta form, an alkali silicate, magnesiumnoxychloride, and magnesium oxysulphate, and mixtures of two or morethereof. The preferred hydraulic binder is gypsum, i.e calcium sulphatehemihydrate in either the alpha or beta form.

The finely divided lignocellulosic fibres are preferably used in anamount of from about 2.5% to about 15%, more preferably from about 3% toabout 12% inclusive by mass of the hydraulic binder.

The finely divided lignocellulosic fibres and the hydraulic binder aremixed either with water or with a solution of water and a polyvinylalcohol. In the latter case, the solution may contain about 1% to about10% inclusive of the polyvinyl alcohol on the mass of the water.

The finely divided lignocellulosic fibres and the hydraulic binder mustbe mixed with sufficient of the water or the water/polyvinyl solution toform a paste.

In step (a) the finely divided lignocellulosic fibres may be dispersedin water or the solution of water and a polyvinyl alcohol, andthereafter the hydraulic binder in dry form may be added thereto andmixed to form the paste.

Alternatively in step (a) the finely divided lignocellulosic fibres indry form may be mixed with the hydraulic binder in dry form, andthereafter water or the solution of water and a polyvinyl alcohol may beadded thereto and mixed to form the paste.

The method of the invention may include a step, between step (a) and (b)of:

(e) immersing particles of a lignocellulosic material in water or in asolution of a polyvinyl alcohol in water, removing the surplus water orsolution, and blending the particles with the paste of step (a).

By particles of a lignocellulosic material there is meant chips, strandsor flakes of a lignocellulosic material, i.e particles which are muchlarger in size than the size of the finely divided lignocellulosicfibres, and which are intended as a filler material and not as atheological controller and foam stabiliser.

The solution of the polyvinyl alcohol in water preferably comprises anamount of from about 1% to about 10% of the polyvinyl alcohol by mass onthe mass of the water.

In step (b), the foam is preferably generated by injecting air into amoving stream of a polyvinyl alcohol solution in water, the solutioncontaining from about 1% to about 18% by mass inclusive of the polyvinylalcohol, on the total mass of the solution, more preferably an amount ofabout 2% to about 7% by mass inclusive of the polyvinyl alcohol on thetotal mass of the solution.

A suitable surfactant may be added to the polyvinyl alcohol solution inorder to lower surface tension and to propagate a fine cellular foam.Suitable surfactants include the silicone glycol copolymers such asDC193 or DC197 by Dow Corning

The amount of foam added relative to the total mass of the paste, i.ethe total mass of the lignocellulosic fibres and particles (if present),the hydraulic binder and water, is at a level of from 0.05% to 15%inclusive, preferably in an amount of from 1.5% to 8% inclusive by mass,more preferably in an amount of from 1.5% to 5% inclusive by mass.

In step (c), the foamed product may be cast or poured or otherwisedispensed into a suitable mould or casting receptacle or the like.

In step (d), the hydraulic binder is allowed to set, with or withoutacceleration promoted by heat induction or catalysis, to form thecomposite product.

Thereafter, the composite product may be used directly, or may be cutinto smaller sections for use.

According to a second aspect of the invention there is provided acomposite product made by the method set out above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph from a test of a solid block of eucalyptus depictingload, applied at right angles to the direction of the grain on the Xaxis, as against displacement on the Y axis; and

FIG. 2 is a graph of a test of a composite product of the invention,depicting load on the X axis as against displacement on the Y axis.

DESCRIPTION OF EMBODIMENTS

The invention relates to a method of making a composite product fromfinely divided lignocellulosic fibres, a hydraulic binder and water,optionally with a polyvinyl alcohol.

The finely divided lignocellulosic fibres may be obtained from anysuitable source, and in particular may be obtained from paper millsludge, waste paper or refined fibres such as are used in medium densityfibreboard manufacture.

The makeup and method of preparation of a paper mill sludge for use inthe method of the invention is set out below.

A typical paper mill sludge has a pH of the saturated paste of 8.13, afibre percentage of 14.4, and results from X-ray emission scans asfollows: Ca 18%; K 0.23%; Cl 0.2%; P 0.15%; S 0.12%; Si 4.4%; Al 3%;Mg0.8%; NaO,0.17%; C 68% and Fe 1.7% This analysis is based onqualitative X-ray emission scans. Hydrocarbon contents are determined bythe burn off method. A typical sludge analysis is as follows:

INORGANIC ANALYSIS % LOI at 600° C. (indicative of total organics) 79.10% Ash at 900° C. 20.74 % Ash % Sample (m/m) (m/m) Calcium as CaO 8.281.72 Aluminum as Al₂O₃ 13.57 2.81 Magnesium as MgO 0.41 0.09 Iron asFe₂O₃ 0.43 0.09 Phosphates as PO₄ ²⁻ ND ND Sulphates as SO₄ ²⁻ 24.004.98 Acid insolubles 52.02 10.65 ND = None detected.

Effluent emerging from a clarifier is typically a sludge of 2% solids inwater. Having passed through a roller press, the sludge is at a 20%solids in water, following upon which it is typically passed through ascrew press and a pelletising screen, whereupon the solids percentagerises from 30% to 60%.

The pellets of sludge are dried thoroughly to 0 to 15% water, preferablym the range of 0 to 3% water.

The form of milling is also an important prerequisite for sludgetreatment. Milling is preferably done in an attritor mill or a platemill or a stone mill, whereby two plates in close proximity to oneanother either, horizontal or vertical, move at disparate speeds one tothe other, typically with one plate stationary although there may becontrary rotation or rotation at differential speeds to one another.Pelletised and dried sludge or paper waste is fed through the centre andis spun outward by centrifugal force and is rolled and the fibresseparated without truncating their length. Clearance between the placesis very small, right down to fibre diameter. All solid agglomerates arebroken to finely divided dry powder. The milling may be referred to asthe action of a stone mill, an attritor mill or a plate mill. Aftermilling fines may be separated from the fibres by air separation ifdesired.

As an alternative, paper mill sludge exiting the paper mill, and withoutany pre-treatment as described above, but taking into account its waterpercentage which may be as high as 65% on a mass basis as it exits thepaper mill, may be let down or slushed in water or a solution of waterand a polyvinyl alcohol, and then the hydraulic binder may be blendedwith the fibre-in-water suspension to form the paste.

In this case, the blending or mixing of the paper mill sludge with thehydraulic binder is of great importance, to ensure that the paper millsludge does not form lumps or clumps, but rather disperses evenlythroughout the hydraulic binder. Suitable mixers are pin or paddle typemixers or high speed dispersers.

Another material for use in the method of the invention is refined fibresuch as is used in medium density fibreboard production—(MDF). MDF isproduced from softwoods or hardwoods. Slab wood or round wood isconverted into chips with a typical dimension of about 20 mm in achipping machine. The chips are then screened to remove under sizedmaterials, such as those below 5 mm, and over sized materials such asthose over 40 mm. The sized chips are then treated to remove adheringdirt or grit and are steamed for several minutes under pressure at atemperature of about 160° C.

Thereafter the steamed chips are forced into the narrow gap between therotating discs of a refiner. An example of such a refiner is a Sunddefibrator. Individual fibre, fibres or fibre bundles are mechanicallyabraded from the surfaces of the steamed, softened chips. These thenpass from the refiner to a drier. For example, in the drier, the wetfibres, including some residual steam, are combined with hot flue gasesfrom a gas burner and the mixture then passed at high velocity along aflash drying tube. At the end of the tube, the dried fibres areseparated from the steam and hot eases in a cyclone and are stored readyfor use.

The finely divided lignocellulosic fibres are mixed with a hydraulicbinder, as described below.

The hydraulic binder may be selected from the group consisting of ahydraulic cement, such as a Portland cement e.g ordinary Portland cementor Rapid Hardening Portland cement, a calcium sulphoaluminate cement, ahigh alumina cement, a gypsum cement, calcium sulphate hemihydrate ineither the alpha or beta form, an alkali silicate, magnesiumoxychloride, and magnesium oxysulphate, and mixtures of two or morethereof.

The preferred hydraulic binder is a Portland cement such as ordinaryPortland cement or Rapid Hardening Portland cement, or a magnesiumoxychloride, or more preferably gypsum, i.e calcium sulphate hemihydratein either the alpha or beta form.

On the grounds of both cost and the fact that they are both wasteproducts. the preferred constituents of the invention are gypsum in theform of calcium sulphate hemihydrate in either the alpha or beta form,paper mill sludge or common mixed paper waste, These combinations alsohave the added advantage of rapid setting and ease of subsequent drying.

In step (a) of the method of the invention, the finely dividedlignocellulosic fibres are mixed with the hydraulic binder and witheither water or a solution of water and a polyvinyl alcohol, to form apaste.

In step (a) the finely divided lignocellulosic fibres may be uniformlydispersed in water or the solution of water and a polyvinyl alcohol, andthereafter the hydraulic binder in dry form may be added thereto andmixed to form the paste.

Alternatively in step (a) the finely divided lignocellulosic fibres indry form may be intimately mixed with the hydraulic binder in dry form,and thereafter water or the solution of water and a polyvinyl alcoholmay be added thereto and mixed to form the paste.

The finely divided lignocellulosic fibres perform the very importantfunction of rheological controller and foam stability agent. It is to benoted that other fibres such as synthetic fibres, e.g polyester fibresand inorganic fibres, generally have an insignificant influence onrheology and are not suitable for use in the method of the presentinvention.

An auxiliary pozzolanic binder of very fine particles size may beincluded in the paste produced in step (a). For example, undensifiedsilica fume may be added to the paste premix in order to add tothixotrophy, to accelerate cement hydration, and to promote thestability of the foamed product as a function of particle size andparticle surface area thus preventing collapse during cement hydration,and to contribute to the strength of the composite product.

For example , undensified or densified silica fume may be added to thecombination of finely divided lignocellulosic fibres and hydraulicbinder in an amount of from about 5% to about 17%, more preferably fromabout 10% a to about 15% by mass, on the mass of the hydraulic binder.

Silica fume has an average particle size of 0.15 micron and a surfacearea of about 20 000 m²/kg. It is amorphous and consists ofapproximately 90% SiO₂.

The method of the invention includes an optional step, step (e). Interms of this step, particles of a lignocellulosic material, i.e largerchips, strands or flakes of a lignocellulosic material, are immersed inwater or in a solution of a polyvinyl alcohol in water, Thereafter, theparticles are removed from the water or solution and the surplus wateror solution is allowed to drain. Then, the particles are blended withthe paste of step (a).

The particles of lignocellulosic material may be for example chips fromthe breakdown saws of a saw mill or chips such as those used in particleboard but ranging in size up to 25 mm in length, flakes, strands, woodwool and particles of appropriate agricultural fibres such as palmleaves, hemp, rice straw, wheat straw and the like.

The polyvinyl alcohol solution in which the particles are immersed,typically contains from 1% to 3% by mass of polyvinyl alcohol on thetotal mass of the solution.

The step of immersing the lignocellulosic particles in the solution ofthe polyvinyl alcohol in water results in the particles beingstrengthened and prevents water uptake by the particles from the pasteitself.

These particles are used as a filler material and not as a rheologicalcontroller or foam stabiliser.

In step (b) of the method of the invention, a foam generated from apolyvinyl alcohol is introduced into the paste and mixed to form afoamed product. In this way, the cellular nature of the final compositeproduct is achieved. This cellular structure controls the density of thecomposite product, imparts good acoustic or sound attenuation propertiesto the composite product, ensures that its mode of failure undercompression, such as in a rock support, is correct, and favourablyinfluences cost, handling and working properties. In particular, thepolyvinyl alcohol foam in combination with finely dividedlignocellulosic fibres improves mechanical properties.

Thus, in the case of cement, the mode of failure in compression, insteadof being catastrophic as would be expected of a cementitious matrix, issuch that as deformation continues, increasing compressive load may becarried. This is comparable to the mode of failure of a hard wood inrock support in deep level mining. In the case of gypsum, an open cellstructure of low density and good cohesion is produced.

The polyvinyl alcohol utilised in step (a) or in step (e) is preferablya low viscosity partially hydrolysed polyvinyl alcohol such as Mowiol4/88 by Clariant. This polyvinyl alcohol serves as a colloidalprotector, stabilising the foam and as a polymer it reinforces thelignocellulosic fibres, to which it has a special affinity.

The polyvinyl alcohol used to form the foam is preferably a higherviscosity polyvinyl alcohol such as Mowiol 18/88 by Clariant. Thispolyvinyl alcohol propagates a dry stable foam compatible with hydraulicbinders which also reinforces the resulting open cellular foam matrix.

Mowiol 4/88 and Mowiol 18/88 have a degree of hydrolysis of 87.7 mol %and an ester value in mg KOH per g of 140, and a residual acetal contentof 10.7 weight percent.

As stated above, the polyvinyl alcohol solution from which the foam isformed preferably contains a surfactant. Suitable surfactants are thesilicone glycol copolymers such as DC193 or DC197 by Dow Corning

In step (c) of the method of the invention, the foamed product is castor poured or otherwise dispensed into a suitable mould or castingreceptacle.

In step (d) of the method of the invention, the hydraulic binder isallowed to set, with or without acceleration promoted by heat induction,or catalysis.

After a sufficient period of time allowing sufficient strength to bedeveloped in the composite product, the product may be used directly ormay be cut into sections for use.

An example of the method of the invention for the production of slabs ofcomposite product for use as a rock, support in a mine will now begiven.

EXAMPLE 1

Step 1

200 kg of wood chips with an average length of 25 mm are blended with200 kg of wood chips with an average length of 10 mm and the mixture issubmerged in a polyvinyl alcohol solution of 3% of Mowiol 4/88 in wateron a mass basis. After a dwell time of one minute in the solution, thewood chips are removed and the excess solution is allowed to drain backinto the tank.

Step 2

1 000 kg of ordinary Portland cement is blended with 100 kg of milledand cleaned paper mill sludge and 150 kg of undensified silica fume. 300kg of water is added and the mixture mixed into a paste.

Step 3

The 400 kg of pre-weighted wood chips from Step 1 are now added andmixed.

Step 4

50 kg of a 3% polyvinyl alcohol solution of 18/88 Mowiol polyvinylalcohol by Hoechst and 100 gms of DC193 surfactant by Dow Corning, allin water, is foamed in a foam generator and is added to the mix andfolded in until a uniform cementitious foam is achieved.

Step 5

The mix is now dispensed into suitable moulds and the cement is allowedto set.

Step 6

Demoulding takes place and cure is allowed to go to completion.

Step 7

The resulting cellular cement bound lignocellulosic composite may eitherbe used in the moulded form or it may be cut easily, e.g with a saw,ready for use.

The dry cured density of the resulting cementitious composite iscontrolled between 300 and 1000 kg/m³ as a function of the amount offoam added. For rock support, the ideal density is between 650 and 1000kg/m³ which gives an initial deformation under a compressive loadbetween 3.5 and 5 MPa, which is what is required for rock support, andthe mode of failure thereafter is non catastrophic,

Referring to the drawings, FIG. 1 is a graph from a test of a solidblock of eucalyptus in which a load was applied to the block at rightangles to the direction of grain. Load is depicted on the X axis asagainst displacement on the Y axis, so illustrating the mode of failureof the block.

FIG. 2 is a graph of the results of a test on a composite product of theinvention, with a load being applied to the product. Again load isdepicted on the X axis as against displacement on the Y axis, and may becompared to the graph of FIG. 1.

The composite product has a superior load support characteristic to thehard wood in that displacement only commences at a compressive load ofapproximately 5 MPa, which in any case can be varied as a function ofdensity, and then continues to support increasing load with gradualdisplacement. The hard wood on the other hand shows displacementcommencing immediately when subjected to load.

A further example of the invention win now be given.

EXAMPLE 2 Preparation of a Gypsum Foam Acoustic Product.

120 kg of common mixed paper waste in a slurry containing 65% by mass ofwater is slushed or dispersed in 1 700 kg of a 2% solution of Mowiol4/88 in water, until the fibres are separate and intimately dispersed. 2000 kg of calcium sulphate hemihydrate in the beta form is blended withthe mix and into the resulting smooth paste is added 250 kg of a foamprepared from a 3% in water solution of Mowiol 18/88, the foam havingbeen generated in an air injection foam generator, to produce a dryfluffy consistency. The foam is folded into the mix. The mix is thenformed into stab form and the calcium sulphate hemihydrate is allowed toset. Setting occurs within 10 minutes and thereafter the formed slabsare dried to give a final density of 325 kg/m³.

In order further to improve the material behaviour in fire, and to addcohesive strength, a 50% in water solution of sodium silicate Code 3379by Silicate and Chemical Industries of South Africa, is applied to theouter surfaces of the slabs, and the slabs are re-dried, the density nowhaving been increased to 375 kg/m³.

What is claimed is:
 1. A method of making a composite product whichincludes the steps of: (a) mixing: (i) a hydraulic binder comprisingcalcium sulphate hemihydrate in either the alpha or beta form; (ii)finely divided lignocellulosic fibres comprising paper mill sludge inamount of from 1% to 30% inclusive by mass of the hydraulic binder; and(iii) water in an amount sufficient to form a paste; (b) introducing afoam generated from a polyvinyl alcohol into the paste, in an amount offrom 0.05% to 15% inclusive by mass of the paste and mixing to form afoamed product; (c) forming the foamed product into a desired shape; and(d) allowing the hydraulic binder to set to form the composite product,the composite product being formed in the absence of a thermosettingresin.
 2. A method according to claim 1 wherein the finely dividedlignocellulosic fibres are present in an amount of from 2.5% to 15%inclusive by mass of the hydraulic binder.
 3. A method according toclaim 1 wherein in step (a) the finely divided lignocellulosic fibresand the hydraulic binder are mixed with a solution of water and apolyvinyl alcohol, containing from 1% to 10% inclusive of the polyvinylalcohol by mass of the water.
 4. A method according to claim 1 whereinin step (a) the finely divided lignocellulosic fibres are dispersed inwater or the solution of water and a polyvinyl alcohol, and thereafterthe hydraulic binder in dry form is added thereto and mixed to form thepaste.
 5. A method according to claim 1 wherein in step (a) the finelydivided lignocellulosic fibres in dry form are mixed with the hydraulicbinder in dry form, and thereafter water or the solution of water and apolyvinyl alcohol is added thereto and mixed to form the paste.
 6. Amethod according to claim 1 where in in step (b) the foam generated froma polyvinyl alcohol is introduced into the paste in an amount of from1.5% to 8% inclusive by mass of the paste.
 7. A method according toclaim 1 wherein in step (b) the foam is generated by injecting air intoa stream of a polyvinyl alcohol solution in water, the solutioncontaining from 1% to 18% inclusive by mass of the polyvinyl alcohol onthe total mass of the solution.
 8. A composite product made by a methodwhich includes the steps of: (a) mixing: (i) a hydraulic bindercomprising calcium sulphate hemihydrate in either the alpha or betaform; (ii) finely divided lignocellulosic fibres comprising paper millsludge in amount of from 1% to 30% inclusive by mass of the hydraulicbinder; and (iii) water optionally containing a polyvinyl alcohol, thewater being present in an amount sufficient to form a paste; (b)introducing a foam generated from a polyvinyl alcohol into the paste, inan amount of from 0.05% to 15% inclusive by mass of the paste and mixingto form a foamed product; (c) forming the foamed product into a desiredshape; and (d) allowing the hydraulic binder to set to form thecomposite product, said composite product being formed in the absence ofa thermosetting resin.
 9. A composite product according to claim 8wherein the finely divided lignocellulosic fibres are present in anamount of from 2.5% to 15% inclusive by mass of the hydraulic binder.10. A composite product according to claim 8 wherein the finely dividedlignocellulosic fibres and the hydraulic binder are mixed with asolution of water and a polyvinyl alcohol, containing from 1% to 10%inclusive of the polyvinyl alcohol by mass of the water.
 11. A compositeproduct according to claim 8 wherein in step (a) the finely dividedlignocellulosic fibres are dispersed in water or the solution of waterand a polyvinyl alcohol, and thereafter the hydraulic binder in dry formis added thereto and mixed to form the paste.
 12. A composite accordingto claim 8 wherein in step (a) the finely divided lignocellulosic fibresin dry form are mixed with the hydraulic binder in dry form, andthereafter water or the solution of water and a polyvinyl alcohol isadded thereto and mixed to form the paste.
 13. A composite productaccording to claim 8 wherein in step (b) the foam generated from apolyvinyl alcohol is introduced into the paste in an amount of from 1.5%to 8% inclusive by mass of the paste.
 14. A composite product accordingto claim 8 wherein in step (b) the foam is generated by injecting airinto a stream of a polyvinyl alcohol solution in water, the solutioncontaining from 1% to 17% inclusive by